ARTICLE pubs.acs.org/IECR
Effect of Demineralization on Product Yield and Composition at Isothermal Pyrolysis of Eynez Lignites Murat Sert,* Levent Ballice, Mithat Y€uksel, and Mehmet Saglam Ege University, Department of Chemical Engineering, Bornova, Izmir, Turkey ABSTRACT: In this study, the effect of the mineral matter of Eynez lignite on the volatile hydrocarbon recovery, the yield of polycyclic aromatic hydrocarbons (asphaltenes, preasphaltenes), and the carbon in the solid residue was investigated. First, lignite was demineralized with HCl, HNO3, and HF, successively. Then, the samples obtained from the demineralization step were pyrolyzed in isothermal pyrolysis reactor. The reaction temperatures are 450, 500, 550, 600, and 650 °C. Pyrolysis products were analyzed in capillary gas chromatography, and product evolution rate was calculated. The results show that removal of the inorganic elements in HCl decreased the volatile hydrocarbon recovery and that HNO3 slightly affected the conversion of organic materials in the pyrolysis reactions. In contrast, removal of the inorganic elements in HF increased the conversion in pyrolysis reactions. This behavior is explained by the inhibitive effect of the silicate minerals.
1. INTRODUCTION Coal is an extraordinarily complex and heterogeneous material whose physical and chemical properties are relatively difficult to determine and usually contains a significant quantity of different inorganic and organic impurities that could cause environmental problems in its usage cycle.1 Lignite, one of the important energy sources, has intensively been used for electricity production. Turkey has a considerable amount of coal reserves. About 80% of the electric energy production in Turkey is provided by thermal power plants which use fossil fuels. Lignite, the most abundant domestic energy source, is consumed in most of these plants. Turkey has approximately 0.85% of the world’s lignite reserves. Lower heat values vary from 1100 to 4500 kcal/kg, and some are even higher. However, their sulfur content is generally high. Soma (Manisa) and Tunc-bilek (K€utahya) lignites are higher in heat content and lower in sulfur content.2,3 Solid fuels are important as energy sources and chemical feedstock worldwide. A growing interest has been especially shown in research in this field in countries such as Turkey which have large solid fuels deposits such as lignites. Turkey, especially in the Tuncbilek and Soma regions, has an estimated reserve of 8.4 billion tones.4 For a long time, Turkish studies have concentrated on these particular lignites as an alternative energy source. These studies were focused on techniques such as pyrolysis and extraction. In these techniques, the structure of solid fuels plays an important role.4 In addition, coals having high ash and sulfur content are unsuitable for efficient use in carbonization, combustion, gasification, and liquefaction. Utilization of such coals leads to environmental pollution and other deleterious effects. The high-quality coal reserves in the world are gradually depleting, and therefore, there is a growing interest to utilize the inferior grades, which contain high ash and high sulfur. It is necessary to demineralize and desulfurize such coals prior to utilization.5 Coal contains variable amounts of mineral matter which includes clay minerals, sulfides, carbonates, sulfates, chlorides, r 2011 American Chemical Society
silicates, oxides, hydroxides, and phosphates. Mineral matter is generally considered to be the sum of all inorganic minerals and elements that are present in coal. Thus, all elements in coal, except organically combined C, H, N, O, and S, are classified by this definition as mineral matter.6 Mineral matter in coal has been classified in different ways, but for practical purposes, it is useful to classify it as inherent or adventitious mineral matter. Inherent mineral matter represents the material that is too closely associated with the coal substance to be readily separated from it by physical methods, and the adventitious represents the material that is less intimately associated with the coal substance and thus more readily removable on coal cleaning.7 Demineralization of coal may be achieved by both physical and chemical methods. The physical methods are based on the difference in the physical properties of the minerals and the carbonaceous part of the coal. The chemical methods are based on solubilization of the inorganic constituents or on their converted forms in various solvents. However, a simple physical separation may not be adequate when mineral grains are tiny and strongly bound to the surrounding carbonaceous material as in some low-grade deepmined bituminous coals; grinding fine enough to liberate mineral grains may be impractical. While some demineralization can be achieved for many fuels by simple physical processes, chemical cleaning is known as being the only solution to obtain clean demineralized coals.8,9 Leaching is one of the most common methods employed for lignite demineralization. Lignite is mixed with alkali or acid, and its mineral content is extracted while being stirred. Various chemical processes are developed employing agents such as HCl, HF, HNO3, and NaOH for the extraction of its mineral content. Received: April 20, 2011 Accepted: July 28, 2011 Revised: July 20, 2011 Published: July 28, 2011 10400
dx.doi.org/10.1021/ie2008604 | Ind. Eng. Chem. Res. 2011, 50, 10400–10406
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Elemental Analysis of Eynez Lignite (wt %) C
H
N
S
ELIG-R
65.6
5.1
1.0
